A Winter Travel Parable

Note: The following story is a work of fiction, but the meteorological principles on which it is built are sound. Any resemblance to real students is purely coincidental, but any resemblance to a real winter storm should be considered purely educational.

The weather during finals week was the best that could be expected, for winter in Oklahoma, at least. A regime of zonal flow (meteorological slang that, at least in our part of the world, equates to westerly winds) ushered in warm, dry days and cool nights over much of the central and southern United States. Travel home for Christmas was no problem for our three protagonists, seniors in meteorology at OU. The trip back would be a different matter.

A week before the end of winter break, the national weather picture was calm and cheery. Though snow had fallen over much of the northern US, bringing a white Christmas from St. Louis to Detroit and points north and west, it had been dry and sunny for several days. Even the synoptic weather charts looked innocuous; the only hint of things to come was a weak 1005 mb low approaching the rocky mountains. The forecast called for a chance of light snow over Minnesota and the Dakotas, but nothing that would stop our three OU students from returning safely to Oklahoma from their homes in Chicago, Denver, and Houston.

Matters changed over the next 48 hours, as the low crossed the rockies and phased with a shortwave trough in the polar jet. Such interactions between weather systems are important and often difficult to predict, adding yet another layer of uncertainty onto the already difficult task of winter weather forecasting. As a result, the emerging storm intensified, and its minimum pressure dropped to 995 mb (as with hurricanes, the lower the central pressure of a winter storm, the stronger the storm is). Fed by an arctic airmass over eastern Alaska and western Canada, a strong arctic cold front began surging southward from the central plains, dropping temperatures in the Dakotas well below zero. Wind flows counter-clockwise around low pressure systems in the northern hemisphere thanks to the Coriolis force, and the lower the pressure of the storm, the stronger the winds. Thanks to these effects, ahead of the storm, strong southerly winds brought warm, moist air north from the gulf of Mexico into much of the central and eastern US.

On the last day of break, our traveler in Denver awoke to very heavy snow. Nine inches were already on the ground, with a foot more expected before it was all said and done. Turning on his computer and pulling up the latest weather charts, he quickly saw why – Denver was to the north and west of the low, putting it in an area of easterly winds. For Denver, easterly winds means upslope flow. Due to the decrease in pressure with altitude, air flowing up a slope cools as it rises (an effect often known as 'adiabatic cooling'). Cooler air has a lower saturation mixing ratio (meaning it can't hold as much water vapor), so excess moisture being brought in by the storm from the Gulf of Mexico was now being dumped along the eastern slope of the rockies in the form of heavy snowfall. It was anyone's guess how long it would take airport officials to clear the snow from runways and get travel moving again.

Our traveler in Chicago awoke praising his good fortune – though the sky was cloudy, the temperature was already in the 40's, and was expected to hit a record high that afternoon. Looking outside, he saw trickles of grimy water along the edges of the streets as the warmth ate away at the remnants of the week-old snow. This snowmelt, along with the warm, moist southerly winds, would be his downfall. After sunset, as temperatures began to drop, a thick fog began to roll in, dropping visibility to less than a hundred feet and canceling all flights in and out of the city. Warm, moist winds over a snowy surface are the perfect conditions for 'advection fog'. Fog forms when near-surface air is cooled to near its dewpoint. When warm, moist gulf air is brought (advected) northward over snow-covered ground, particularly at night, the near-surface air is cooled by the snow, resulting in thick fog.

In Houston, there was not a cloud in the sky, yet our third unfortunate protagonist found herself sitting in the Houston airport, her flight having just been delayed for the fourth time in as many hours. Though Houston was well within the warm sector of the low, and conditions outside were sunny with temperatures in the low 80's, our traveler's flight from Houston to Oklahoma City couldn't depart, because the plane that was to service that flight was stuck on the ground in the heavy snow in Denver. Because of the nature of the airport network, particularly during the busy holiday travel season, there are rarely extra planes available to stand in for those stuck on the ground elsewhere.

For the sake of a happy ending, we'll say that all three made it back safely in time for the start of classes, but be mindful of the moral of this story: winter travel is full of unforseen hazards. Winter weather is notoriously difficult to predict, with just one or two degrees making the difference between an inch of rain and a foot of snow. Also, it's not just the snow and ice that one must worry about, but fog, wind, and cold too. Then keep in mind that all these winter weather phenomena are strongly tied to local effects, such as topography and snow cover, and take into account the interconnected nature of our modern travel network, and it's no wonder that winter travel can be such a headache – but also a chance to put meteorological theory into practice.





Story is © Nate Snook, 2009